I think you may have misunderstood the district's use of the term "eligible". While a diagnosis of dyslexia may represent an eligible condition under general section 504 guidelines, the student is only truly eligible for a 504 plan if the eligible condition also causes a substantial "need" (i.e. substantially limiting access to education). For almost all school districts the threshold of need is determined by grades and standardized test scores. In general, if a student is maintaining an average GPA and average range test scores, he/she would not be demonstrating a need and therefore not entitled to a 504 plan.